1. Field of the Invention
This invention relates to a circuit board having a new structure which includes a capacitor structure in itself.
2. Description of the Related Art
Regarding electric/electronic devices, multifunctionalization and reduction in size, weight and thickness have been advancing rapidly. In this connection, regarding circuit boards to be incorporated in such devices, there is a demand for such circuit boards that allow higher-density mounting of components and have a more multilayered structure.
To meet this demand, for example, efforts have been being made to provide a circuit board arranged such that the board itself has functions of components which are normally mounted, to reduce the number of components mounted on the surface of the circuit board to thereby allow the circuit board to have a smaller mounting surface.
Regarding a circuit board having the function of a capacitor in itself, for example, technology disclosed in PCT Application Published Japanese Translation No. Hei 5-500136 is known.
The circuit board disclosed in this prior art document has the following structure: Within an insulating substrate having a certain plane size and a certain thickness, two conductive layers having the same plane size as the insulating substrate are arranged one over the other in the direction of the thickness of the insulating substrate, parallel to each other with a predetermined distance between them, and a layer of a dielectric material is inserted between the two conductive layers.
Thus, in this circuit board, the insulating substrate contains a parallel plate capacitor structure which is formed of the two parallel conductive layers functioning as electrodes and the dielectric-material layer inserted between them and which has the same area as the insulating substrate.
The capacitor structure in this prior-art document has, however, the following problems:
First, it is difficult to increase the capacitance of the capacitor structure contained.
In the above-described structure, provided that the upper and lower conductive layers have a fixed area, the capacitance of the capacitor structure can be increased by making the distance between the conductive layers smaller.
Making the distance between the conductive layers smaller means making the dielectric-material layer inserted between the conductive layers uniformly thinner. However, practically, it is difficult to make the dielectric-material layer uniformly thinner. Hence, the dielectric-material layer formable has a thickness limit.
According to common knowledge of those skilled in the art, the above-described structure is formed by, for example, spin-coating the surface of the lower conductive layer with the dielectric material and then forming the upper conductive layer on the dielectric-material coating.
However, generally, the thinner the coating to be formed, the more difficult it is to form the coating with a uniform thickness. Hence, in order to ensure that the coating formed has a uniform thickness, the coating needs to have a certain measure of thickness.
Second, the dielectric-material layer is far lower in strength than the insulating substrate which is normally made of a composite material such as glass fiber reinforced epoxy resin. Since the dielectric-material layer of such low strength extends within the insulating substrate over its transverse cross-sectional area, the insulating substrate as a whole has a lowered strength and cannot provide sufficient reliability when used as a circuit board.
The third problem relates to the fact that the dielectric-material layer is formed with the same plane size as the insulating substrate.
Practically, there are cases such that it is desired to connect a capacitor only with a certain component mounted on the surface of the circuit board. In such cases, it is enough to form a capacitor structure near that particular component, locally.
However, in the above-described circuit board, the capacitor structure has to be formed within the insulating substrate, over the transverse cross-sectional area of the insulating substrate. Hence, only a part of the capacitor structure formed is used effectively, and the other part thereof is useless.